1 2Concurrency Managed Workqueue (cmwq)
3 4September, 2010 Tejun Heo <tj@kernel.org>
5 Florian Mickler <florian@mickler.org>
6 7CONTENTS
8 91. Introduction
102. Why cmwq?
113. The Design
124. Application Programming Interface (API)
135. Example Execution Scenarios
146. Guidelines
157. Debugging
16 17 181. Introduction
19 20There are many cases where an asynchronous process execution context
21is needed and the workqueue (wq) API is the most commonly used
22mechanism for such cases.
23 24When such an asynchronous execution context is needed, a work item
25describing which function to execute is put on a queue. An
26independent thread serves as the asynchronous execution context. The
27queue is called workqueue and the thread is called worker.
28 29While there are work items on the workqueue the worker executes the
30functions associated with the work items one after the other. When
31there is no work item left on the workqueue the worker becomes idle.
32When a new work item gets queued, the worker begins executing again.
33 34 352. Why cmwq?
36 37In the original wq implementation, a multi threaded (MT) wq had one
38worker thread per CPU and a single threaded (ST) wq had one worker
39thread system-wide. A single MT wq needed to keep around the same
40number of workers as the number of CPUs. The kernel grew a lot of MT
41wq users over the years and with the number of CPU cores continuously
42rising, some systems saturated the default 32k PID space just booting
43up.
44 45Although MT wq wasted a lot of resource, the level of concurrency
46provided was unsatisfactory. The limitation was common to both ST and
47MT wq albeit less severe on MT. Each wq maintained its own separate
48worker pool. A MT wq could provide only one execution context per CPU
49while a ST wq one for the whole system. Work items had to compete for
50those very limited execution contexts leading to various problems
51including proneness to deadlocks around the single execution context.
52 53The tension between the provided level of concurrency and resource
54usage also forced its users to make unnecessary tradeoffs like libata
55choosing to use ST wq for polling PIOs and accepting an unnecessary
56limitation that no two polling PIOs can progress at the same time. As
57MT wq don't provide much better concurrency, users which require
58higher level of concurrency, like async or fscache, had to implement
59their own thread pool.
60 61Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
62focus on the following goals.
63 64* Maintain compatibility with the original workqueue API.
65 66* Use per-CPU unified worker pools shared by all wq to provide
67 flexible level of concurrency on demand without wasting a lot of
68 resource.
69 70* Automatically regulate worker pool and level of concurrency so that
71 the API users don't need to worry about such details.
72 73 743. The Design
75 76In order to ease the asynchronous execution of functions a new
77abstraction, the work item, is introduced.
78 79A work item is a simple struct that holds a pointer to the function
80that is to be executed asynchronously. Whenever a driver or subsystem
81wants a function to be executed asynchronously it has to set up a work
82item pointing to that function and queue that work item on a
83workqueue.
84 85Special purpose threads, called worker threads, execute the functions
86off of the queue, one after the other. If no work is queued, the
87worker threads become idle. These worker threads are managed in so
88called thread-pools.
89 90The cmwq design differentiates between the user-facing workqueues that
91subsystems and drivers queue work items on and the backend mechanism
92which manages thread-pools and processes the queued work items.
93 94The backend is called gcwq. There is one gcwq for each possible CPU
95and one gcwq to serve work items queued on unbound workqueues. Each
96gcwq has two thread-pools - one for normal work items and the other
97for high priority ones.
98 99Subsystems and drivers can create and queue work items through special
100workqueue API functions as they see fit. They can influence some
101aspects of the way the work items are executed by setting flags on the
102workqueue they are putting the work item on. These flags include
103things like CPU locality, reentrancy, concurrency limits, priority and
104more. To get a detailed overview refer to the API description of
105alloc_workqueue() below.
106 107When a work item is queued to a workqueue, the target gcwq and
108thread-pool is determined according to the queue parameters and
109workqueue attributes and appended on the shared worklist of the
110thread-pool. For example, unless specifically overridden, a work item
111of a bound workqueue will be queued on the worklist of either normal
112or highpri thread-pool of the gcwq that is associated to the CPU the
113issuer is running on.
114 115For any worker pool implementation, managing the concurrency level
116(how many execution contexts are active) is an important issue. cmwq
117tries to keep the concurrency at a minimal but sufficient level.
118Minimal to save resources and sufficient in that the system is used at
119its full capacity.
120 121Each thread-pool bound to an actual CPU implements concurrency
122management by hooking into the scheduler. The thread-pool is notified
123whenever an active worker wakes up or sleeps and keeps track of the
124number of the currently runnable workers. Generally, work items are
125not expected to hog a CPU and consume many cycles. That means
126maintaining just enough concurrency to prevent work processing from
127stalling should be optimal. As long as there are one or more runnable
128workers on the CPU, the thread-pool doesn't start execution of a new
129work, but, when the last running worker goes to sleep, it immediately
130schedules a new worker so that the CPU doesn't sit idle while there
131are pending work items. This allows using a minimal number of workers
132without losing execution bandwidth.
133 134Keeping idle workers around doesn't cost other than the memory space
135for kthreads, so cmwq holds onto idle ones for a while before killing
136them.
137 138For an unbound wq, the above concurrency management doesn't apply and
139the thread-pools for the pseudo unbound CPU try to start executing all
140work items as soon as possible. The responsibility of regulating
141concurrency level is on the users. There is also a flag to mark a
142bound wq to ignore the concurrency management. Please refer to the
143API section for details.
144 145Forward progress guarantee relies on that workers can be created when
146more execution contexts are necessary, which in turn is guaranteed
147through the use of rescue workers. All work items which might be used
148on code paths that handle memory reclaim are required to be queued on
149wq's that have a rescue-worker reserved for execution under memory
150pressure. Else it is possible that the thread-pool deadlocks waiting
151for execution contexts to free up.
152 153 1544. Application Programming Interface (API)
155 156alloc_workqueue() allocates a wq. The original create_*workqueue()
157functions are deprecated and scheduled for removal. alloc_workqueue()
158takes three arguments - @name, @flags and @max_active. @name is the
159name of the wq and also used as the name of the rescuer thread if
160there is one.
161 162A wq no longer manages execution resources but serves as a domain for
163forward progress guarantee, flush and work item attributes. @flags
164and @max_active control how work items are assigned execution
165resources, scheduled and executed.
166 167@flags:
168 169 WQ_NON_REENTRANT
170 171 By default, a wq guarantees non-reentrance only on the same
172 CPU. A work item may not be executed concurrently on the same
173 CPU by multiple workers but is allowed to be executed
174 concurrently on multiple CPUs. This flag makes sure
175 non-reentrance is enforced across all CPUs. Work items queued
176 to a non-reentrant wq are guaranteed to be executed by at most
177 one worker system-wide at any given time.
178 179 WQ_UNBOUND
180 181 Work items queued to an unbound wq are served by a special
182 gcwq which hosts workers which are not bound to any specific
183 CPU. This makes the wq behave as a simple execution context
184 provider without concurrency management. The unbound gcwq
185 tries to start execution of work items as soon as possible.
186 Unbound wq sacrifices locality but is useful for the following
187 cases.
188 189 * Wide fluctuation in the concurrency level requirement is
190 expected and using bound wq may end up creating large number
191 of mostly unused workers across different CPUs as the issuer
192 hops through different CPUs.
193 194 * Long running CPU intensive workloads which can be better
195 managed by the system scheduler.
196 197 WQ_FREEZABLE
198 199 A freezable wq participates in the freeze phase of the system
200 suspend operations. Work items on the wq are drained and no
201 new work item starts execution until thawed.
202 203 WQ_MEM_RECLAIM
204 205 All wq which might be used in the memory reclaim paths _MUST_
206 have this flag set. The wq is guaranteed to have at least one
207 execution context regardless of memory pressure.
208 209 WQ_HIGHPRI
210 211 Work items of a highpri wq are queued to the highpri
212 thread-pool of the target gcwq. Highpri thread-pools are
213 served by worker threads with elevated nice level.
214 215 Note that normal and highpri thread-pools don't interact with
216 each other. Each maintain its separate pool of workers and
217 implements concurrency management among its workers.
218 219 WQ_CPU_INTENSIVE
220 221 Work items of a CPU intensive wq do not contribute to the
222 concurrency level. In other words, runnable CPU intensive
223 work items will not prevent other work items in the same
224 thread-pool from starting execution. This is useful for bound
225 work items which are expected to hog CPU cycles so that their
226 execution is regulated by the system scheduler.
227 228 Although CPU intensive work items don't contribute to the
229 concurrency level, start of their executions is still
230 regulated by the concurrency management and runnable
231 non-CPU-intensive work items can delay execution of CPU
232 intensive work items.
233 234 This flag is meaningless for unbound wq.
235 236@max_active:
237 238@max_active determines the maximum number of execution contexts per
239CPU which can be assigned to the work items of a wq. For example,
240with @max_active of 16, at most 16 work items of the wq can be
241executing at the same time per CPU.
242 243Currently, for a bound wq, the maximum limit for @max_active is 512
244and the default value used when 0 is specified is 256. For an unbound
245wq, the limit is higher of 512 and 4 * num_possible_cpus(). These
246values are chosen sufficiently high such that they are not the
247limiting factor while providing protection in runaway cases.
248 249The number of active work items of a wq is usually regulated by the
250users of the wq, more specifically, by how many work items the users
251may queue at the same time. Unless there is a specific need for
252throttling the number of active work items, specifying '0' is
253recommended.
254 255Some users depend on the strict execution ordering of ST wq. The
256combination of @max_active of 1 and WQ_UNBOUND is used to achieve this
257behavior. Work items on such wq are always queued to the unbound gcwq
258and only one work item can be active at any given time thus achieving
259the same ordering property as ST wq.
260 261 2625. Example Execution Scenarios
263 264The following example execution scenarios try to illustrate how cmwq
265behave under different configurations.
266 267 Work items w0, w1, w2 are queued to a bound wq q0 on the same CPU.
268 w0 burns CPU for 5ms then sleeps for 10ms then burns CPU for 5ms
269 again before finishing. w1 and w2 burn CPU for 5ms then sleep for
270 10ms.
271 272Ignoring all other tasks, works and processing overhead, and assuming
273simple FIFO scheduling, the following is one highly simplified version
274of possible sequences of events with the original wq.
275 276 TIME IN MSECS EVENT
277 0 w0 starts and burns CPU
278 5 w0 sleeps
279 15 w0 wakes up and burns CPU
280 20 w0 finishes
281 20 w1 starts and burns CPU
282 25 w1 sleeps
283 35 w1 wakes up and finishes
284 35 w2 starts and burns CPU
285 40 w2 sleeps
286 50 w2 wakes up and finishes
287 288And with cmwq with @max_active >= 3,
289 290 TIME IN MSECS EVENT
291 0 w0 starts and burns CPU
292 5 w0 sleeps
293 5 w1 starts and burns CPU
294 10 w1 sleeps
295 10 w2 starts and burns CPU
296 15 w2 sleeps
297 15 w0 wakes up and burns CPU
298 20 w0 finishes
299 20 w1 wakes up and finishes
300 25 w2 wakes up and finishes
301 302If @max_active == 2,
303 304 TIME IN MSECS EVENT
305 0 w0 starts and burns CPU
306 5 w0 sleeps
307 5 w1 starts and burns CPU
308 10 w1 sleeps
309 15 w0 wakes up and burns CPU
310 20 w0 finishes
311 20 w1 wakes up and finishes
312 20 w2 starts and burns CPU
313 25 w2 sleeps
314 35 w2 wakes up and finishes
315 316Now, let's assume w1 and w2 are queued to a different wq q1 which has
317WQ_CPU_INTENSIVE set,
318 319 TIME IN MSECS EVENT
320 0 w0 starts and burns CPU
321 5 w0 sleeps
322 5 w1 and w2 start and burn CPU
323 10 w1 sleeps
324 15 w2 sleeps
325 15 w0 wakes up and burns CPU
326 20 w0 finishes
327 20 w1 wakes up and finishes
328 25 w2 wakes up and finishes
329 330 3316. Guidelines
332 333* Do not forget to use WQ_MEM_RECLAIM if a wq may process work items
334 which are used during memory reclaim. Each wq with WQ_MEM_RECLAIM
335 set has an execution context reserved for it. If there is
336 dependency among multiple work items used during memory reclaim,
337 they should be queued to separate wq each with WQ_MEM_RECLAIM.
338 339* Unless strict ordering is required, there is no need to use ST wq.
340 341* Unless there is a specific need, using 0 for @max_active is
342 recommended. In most use cases, concurrency level usually stays
343 well under the default limit.
344 345* A wq serves as a domain for forward progress guarantee
346 (WQ_MEM_RECLAIM, flush and work item attributes. Work items which
347 are not involved in memory reclaim and don't need to be flushed as a
348 part of a group of work items, and don't require any special
349 attribute, can use one of the system wq. There is no difference in
350 execution characteristics between using a dedicated wq and a system
351 wq.
352 353* Unless work items are expected to consume a huge amount of CPU
354 cycles, using a bound wq is usually beneficial due to the increased
355 level of locality in wq operations and work item execution.
356 357 3587. Debugging
359 360Because the work functions are executed by generic worker threads
361there are a few tricks needed to shed some light on misbehaving
362workqueue users.
363 364Worker threads show up in the process list as:
365 366root 5671 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/0:1]
367root 5672 0.0 0.0 0 0 ? S 12:07 0:00 [kworker/1:2]
368root 5673 0.0 0.0 0 0 ? S 12:12 0:00 [kworker/0:0]
369root 5674 0.0 0.0 0 0 ? S 12:13 0:00 [kworker/1:0]
370 371If kworkers are going crazy (using too much cpu), there are two types
372of possible problems:
373 374 1. Something beeing scheduled in rapid succession
375 2. A single work item that consumes lots of cpu cycles
376 377The first one can be tracked using tracing:
378 379 $ echo workqueue:workqueue_queue_work > /sys/kernel/debug/tracing/set_event
380 $ cat /sys/kernel/debug/tracing/trace_pipe > out.txt
381 (wait a few secs)
382 ^C
383 384If something is busy looping on work queueing, it would be dominating
385the output and the offender can be determined with the work item
386function.
387 388For the second type of problems it should be possible to just check
389the stack trace of the offending worker thread.
390 391 $ cat /proc/THE_OFFENDING_KWORKER/stack
392 393The work item's function should be trivially visible in the stack
394trace.
395